1. Field of the Invention
The present invention relates to a control device of an internal combustion engine and particularly to a control device for performing a start time control of an internal combustion engine having a plurality of cylinders.
2. Description of the Background Art
In an internal combustion engine, if an air-fuel ratio in an engine cylinder becomes excessively rich during operation, a large quantity of unburned hydrocarbon (HC) is generated. If the air-fuel ratio becomes excessively lean, combustion flame is not propagated thereby causing misfire, and a large quantity of unburned HC is generated in this case, too. In other words, in order to suppress generation of the unburned HC, it is necessary to maintain the air-fuel ratio at a stoichiometric air-fuel ratio or at a slightly leaner ratio. This also holds true for the engine start time.
On the other hand, if fuel injection is carried out at the engine start time, a large quantity of injected fuel adheres in a form of liquid to an inner wall face of an intake port in a case of an internal combustion engine where the fuel is injected into the intake port. Therefore, the air-fuel mixture lead into the cylinder is formed of a part of the injected fuel. Even in a case of an internal combustion engine where the fuel is directly injected into the cylinder, the injected fuel adheres in a form of liquid to a top face of the piston and an inner wall face of the cylinder.
The fuel is gradually vaporized to form an air-fuel mixture by the time an intake valve closes in an intake stroke in a case of the fuel that has adhered to the inner wall face of the intake port, and by the time the piston reaches a top dead center in a compression stroke in a case of the fuel that has adhered to the top face of the piston. This air-fuel mixture makes up a sizable proportion of the whole air-fuel mixture formed in the engine cylinder. Therefore, the air-fuel ratio of the air-fuel mixture formed in the engine cylinder is seriously affected by a quantity of fuel vaporized from the wall face.
In the case of the internal combustion engine where the fuel is directly injected into the cylinder, a quantity of fuel vaporized from the wall face is proportional to the time that elapsed before the piston moves from a bottom dead center to a vicinity of a compression top dead center. The shorter this time is, the smaller quantity of fuel is vaporized from the wall face. On the other hand, the time that elapsed before the piston reaches a vicinity of a compression top dead center is inversely proportional to an engine speed. Also in the case of the internal combustion engine where the fuel is injected into the intake port, the higher the engine speed is, the shorter the time that elapses between injection and the end of intake becomes. Therefore, the higher the engine speed is, the smaller quantity of fuel is vaporized from the wall face. The higher the engine speed is, the larger quantity of fuel is required to be injected.
Japanese Patent Application Laid-open No. 2004-068621 discloses a technique of increasing an injection quantity of fuel injected in order into respective cylinders in the first cycle because the engine speed gradually increases at the start time.
In recent years, a hybrid automobile has been receiving much attention as an environment-friendly automobile. The hybrid automobile is an automobile powered by a conventional engine and by a direct-current power supply, an inverter, and a motor driven by the inverter. In other words, the engine is driven to obtain mechanical power and direct voltage from the direct-current power supply is converted to alternating voltage by the inverter, and the motor is rotated by the alternating voltage obtained by conversion to thereby obtain mechanical power.
Among this type of vehicles, there is one in which an engine is cranked by using a motor generator that has greater driving performance than a normal starter motor and functions also as a generator. In such a vehicle, the engine speed is already high (e.g., 800 rpm) when the fuel injection is started at a time of cranking of the internal combustion engine.
At a start time of the internal combustion engine, two factors, i.e., vaporization time and negative pressure in the intake port control the vaporization of the fuel. There is almost no difference in the vaporization time from one cylinder to another when the engine speed is already high, and therefore the negative pressure in each cylinder affects vaporization more than the difference in the vaporization time from one cylinder to another. In this case, higher negative pressure (lower pressure) is advantageous to vaporization of the fuel.
At the start time of such a vehicle, because the negative pressure in the intake port is in a transient state, the later cylinder in the order of injection has an advantage in vaporization of the fuel and requires a smaller quantity of injection contrary to the technique disclosed in Japanese Patent Application Laid-open No. 2004-068621.
It is also conceivable that the fuel injection is started after the engine is rotated by the motor generator and then the negative pressure is stabilized. However, the fuel is discharged from a fuel injection valve into the port when the vehicle is left unattended for a long time. In this case, such fuel is discharged from the engine without being burned. As a result, an emission quantity of unburned HC is rather increased in some cases.